The present invention pertains to milling of ceramic powders and an improved process for the same.
Ceramic powders are important in a variety of different fields. Examples include manganese zinc ferrites, aluminum nitride, zinc oxide, silicon dioxide, barium titanate, and iron oxide.
Manganese zinc ferrites (Mn,Zn,Fe)Fe2O4 or MZF) are important ceramic materials for the manufacture of ferromagnetic devices including inductors and transformers. Conventionally, commercial MZF are prepared through calcination of mixtures of the single component precursor metal carbonates or oxides followed by milling to the desired particle size range. The milling operation usually involves suspension of the calcined, aggregated material in water to promote a higher milling efficiency than that encountered with dry milling. Aluminum nitride (AIN3) is commonly used in circuit substrates. Zinc oxide is used as a varistor material, and, of course, uses for silicon dioxide ceramics are ubiquitous. Barium titanate is often used in multi-layer capacitors and in piezoelectric devices such as transducers and ultrasonic vibrators.
Each of these ceramics presents its own characteristic problems when attempts are made to deposit those at a fine micron particle size level for their common ceramic uses as above-mentioned. For example, manganese zinc iron ferrite is often made by mixing a precursor mix of magnesium carbonate, zinc carbonate, iron oxide, which is then calcined at about 1000xc2x0 C. The chemicals xe2x80x9chomogenizexe2x80x9d during the calcination process at the high temperature, but this results in primary particles which have sintered. Aluminum nitrides, as commercially prepared, often result in a polycrystalline aggregate which is made by chemical vapor deposition, but to be useful, the aggregate needs to be broken up, i.e., milled. Zinc oxide ceramic typically comes from calcining a mixture of zinc hydroxide and zinc carbonate or the single precursors, and the resultant product, too, needs milling to be useful. Silicon dioxide, as commercially provided, comes from chemical vapor deposition, but while it is very fine, it often aggregates in the process, and therefore also needs milling. Barium titanate is prepared by calcining either BaCO3 and TiO2 or a precursor such as (BaTiO)(C2O4)2xc2x74H2O, which results in polycrystalline aggregates. All are in need of further treatment to make satisfactory submicron particles for their various ceramic material uses.
As an example, the importance of being able to produce thinner dielectric layers is becoming increasingly recognized by the producers of multilayer capacitors (MLC""s) due to end user requirements of reduced size and cost. These capacitors are typically manufactured by co-firing, i.e., sintering alternating multilayers of the ceramic dielectric formulation and a conductive electrode material in a controlled atmosphere at a temperature in the range of about 1000xc2x0 to 1400xc2x0 C.
Dielectric layers have traditionally been produced by preparing a suspension of ceramic powder in a liquid vehicle, usually containing a dispersant, and then adding an organic resin matrix which functions to bind the ceramic particles after tape formation and drying. A variety of methods are known for applying the suspension and binder mixture (hereafter defined as slip) to a substrate to form very thin layers of the suspended solids. Methods such as wet coating, tape-casting (casting), or doctor-blading are readily known to those skilled in the art. The thin, dried layers generally termed as green layers, may then be coated with conductive electrodes and stacked together with similar layers to form a green body. The stack is then trimmed and co-fired to produce a structure consisting of alternating layers of sintered electrode and dielectric which is finally leaded with end terminations to form the finished capacitor. Suspensions used for dielectric compositions in the past have used both aqueous and organic liquids, but because of the environmental and safety concerns, the tendency of late has been to increase the use of aqueous suspensions, which are therefore preferred.
Another trend in the capacitor industry has been to make the dielectric layers thinner to obtain more capacitance per unit volume. Therefore, the thickness of dielectric layers has been reduced, e.g., from 25 microns to 10 microns. It is now desirable to reduce the thickness even less to, for example, 5 microns or less. These thinner layers necessitate the use of extremely small solid ceramic particles in the suspension to produce the required high density and fine grain size in the final fired layer. When ceramic powders are reduced to such small particle sizes, i.e., less than 0.5 microns, they tend to have a significant soluble portion that dissolves in an aqueous suspension, thus causing chemical reactions with the dispersants and binders in solution which may create process problems.
Then too, ever smaller particles are also more difficult to handle, making automated systems unduly complex and expensive.
Barium titanate, the base material of choice for capacitor formulations due to its dielectric characteristics, forms a soluble barium cation in aqueous conditions. The organic additive typically used in the processing contains chemical groups that can react with the soluble cation or its companion hydroxyl ion. Agglomerates of polymer and phase separation or xe2x80x9csalting outxe2x80x9d or precipitation of the metal cation organic complex can take place. These complexes or agglomerates often create voids in the ceramic body during the binder burnout phase prior to sintering and can result in either elevated levels of electrical leakage or electrical shorting paths and/or a deletion effect on the strength of the ceramic. Void formation is particularly unforgiving in layers having a thickness of less than 10 microns and must be eliminated.
Another problem that occurs when making aqueous suspensions with ceramic powders of less than 0.5 microns in diameter is that both the interfacial area between the solids and the liquid carrier and the number of particles in a given volume are greatly increased. This results in a high physical chemical interaction between the solid particles in the liquid phase, and diminished processability, especially at commercially acceptable solids loading levels. Hence, often the benefit of finer particle sizes needs to be countered by the necessity of going to lower solids loadings in the suspensions or slips. Manufacturing processes which expose the suspension to high shear conditions, such as those encountered in pumping or tape casting, result in excessive gelling, and in the worst case, unworkable suspensions with shear thickening characteristics and high viscosities. There is a continuing need to solve these problems.
A variety of attempts have been made to prepare finely divided ceramic powders in aqueous suspensions and slips. For example, U.S. Pat. No. 3,496,008 discloses the ball milling of a ferroelectric material such as barium titanate in a 60% by weight solids loading level of milled material to water. The mixed suspension is rediluted to a desirable consistency for spray application. In U.S. Pat. No. 3,551,197 a dielectric composition is prepared with between 40 to 90 weight percent of a ceramic powder in water. The ceramic powder is selected from a group including barium titanate, strontium titanate, calcium titanate, and lead titanate, and has a particle size of 0.5 to 3 micron. The suspended ceramic material is combined with a binder such as polymethylene glycol or diethylene glycol, for example.
In U.S. Pat. No. 4,968,460 an aqueous emulsion of water soluble polymeric binder is combined with an aqueous suspension of ceramic material in a solids loading of at least 50 weight percent. The polymeric binder is used in a range of 0.5 to 35 weight percent and optionally with up to 5 weight percent of a selected dispersing agent. Tapes prepared from the slip composition had a thickness of between 30 microns and 2.540 mm. Particle sizes in the range of 0.5 to 12 micron are disclosed.
These references, however, do not address the problems encountered in the preparation of aqueous suspensions or slips of ceramic powders having very fine particles of less than 0.5 micron in diameter.
A suspension of ceramic powder having a diameter of 0.5 micron or less which remains suspended in an aqueous carrier fluid for extended periods of time in a substantially unagglomerated state and which maintains an apparent viscosity of less than 3000 centipoise (cps) without solidifying when determined from high shear rates of between 50 to 100/sec, would be a desirable improvement in the art of ceramic suspensions, slips, and the processes for producing them. This is achieved in the present process.
In particular, there are two problems to be resolved to enhance the milling of fine ceramic powders: First, control over the state of surface passivation to control the levels of soluble species during milling. Second, control over solution parameters such as pH and dispersant levels. As earlier discussed, many metal oxide powders are prepared by mixing precursor metal oxides or salts followed by calcination to achieve solid state mixing of the metals and oxygen. However, aggregation of the primary particles occurs during the calcination. To reduce the aggregated particles to the primary particle size requires milling.
Milling is a common powder process used in almost all facets of powder industries, including mineral recovery and processing, ceramic and metal powder processing. Application areas for such powders include industries as diverse as cosmetics, herbicides, metals, ceramics, and pharmaceutical powders. However, milling is a relatively energy inefficient process. For example, it has been estimated that only about 1% of the energy input during milling is actually used to comminute the particulate material.
Wet milling in water is the most desirable approach because of greater efficiency in wet milling and lower toxicity and cost of aqueous-based processing schemes. Since milling efficiency is better for low viscosity slurries, additives for wet mill slurries are usually present that act as dispersants and/or modifiers of the particle surface charge. For example, the solution pH is often controlled to avoid the isoelectric point of the powder where there is no net surface charge to provide an electrostatic barrier to agglomeration. Organic dispersants are also typically added to provide viscosity control.
As fracture takes place during milling, the fresh surfaces created react with the solution phase. As milling proceeds and new surface is created, the solution pH can change, and/or the reservoir of dispersant or other additives becomes increasingly depleted due to the reactions at the surface. It is, therefore, a common practice during milling for the mill to be stopped and the pH adjusted or the dispersant level increased to obtain the initial desirable low viscosity in the milling slurry.
For particles greater than about 10 microns in diameter, the actual surface area presented to the solution phase is too small to measurably deplete the solution components, so dramatic equilibrium changes are not a problem. However, for particulates in which the goal of milling is to obtain submicron particles, the large surface areas created during the milling process can appreciably alter the solution chemistry, and one can lose control over the viscosity. Milling rate and efficiency can be diminished as a consequence.
Another issue is in the aqueous milling of water sensitive materials such as Bi modified-ZnO, BaTiO3 and other AxByOz compounds, non-oxide materials such as aluminum nitride (AIN) and silicon nitride (Si3N4) and even simple metal oxides such as a-Al2O3 in which the phase to be milled undergoes undesirable reactions with water. For example, in the case of ZnO, the material usually converts to the thermodynamically stable Zn(OH)2 and the bismuth oxide phase dissolves completely. Therefore, milling of Bi-modified ZnO is generally conducted in more toxic and expensive non-aqueous solvents such as acetone, methylethylketone, or toluene. In the case of BaTiO3, and similar AxByOz compounds, the Ba2+ leaches out of the particle surfaces to form the more thermodynamically stable, but less desirable BaCO3. The BaCO3 and the resulting Ti-rich BaTiO3 surfaces can lead to loss of grain size control critical in preparing capacitors with specific electronic properties. The a-Al2O3 undergoes surface hydrolysis reactions that also lead to loss of grain size control during subsequent sintering processes. Likewise, manganese zinc ferrites undergo dissolution of the manganese ion and the zinc ion, ultimately leading to loss of grain size control. However, as has recently been shown, many water-sensitive materials can be chemically passivated with simple organic salts. The passivation-dispersion approach, however, has not been applied to materials during milling. It would be expected that as milling proceeds with fresh surface being generated continually requiring passivation that the passivating agent would become rapidly depleted. Thus, passivation would be ineffective in milling operations to produce submicron particles as commonly practiced today. The pH of the suspension also changes with so much new surface continually being exposed.
Accordingly, it is a primary objective of the present invention to make ceramic milling more efficient to produce highly desirable submicron ceramic surfaces, and do so in less time. As an additional benefit, energy consumption is decreased.
An additional objective and result of the present invention is to provide a very thin metal organic salt coating on the ceramic particles which will produce passivation layer minimizing dissolution, and in concert with suitable dispersants, if required, producing a zeta potential which weakens the inherent interparticle attraction. This is effected by dynamic monitoring of the passivating agent during wet milling.
The method of achieving this primary objective as well as others will become apparent from the below-presented detailed description of the preferred embodiment.
An improved wet milling process for ceramic powders that results in materially-enhanced surfaces on the ceramic powder particles and less energy consumption in the milling process. This is achieved by a process that provides dynamic monitoring solution constituent concentration, passivating agent concentration, and careful control of pH with adjustments as necessary, as the wet milling process is occurring. This dynamic or constant monitoring and real time adjustment during the milling process results in substantially improved ceramic powders.